Earthing Type

[eskimo39]

Hi guys, just got home from a job today at a golf course and am puzzled by the earthing arrangement.

The building is fed via 3 overhead powerlines that go to a transformer on a wooden pole approximatly 15 metres fromwhere it enters the building.
The 3 phase incommers go through a large green metal box filled with tar and then into a current transformer before going into a 300a 3P main switch and then onto a large C.U which in turn feeds many other sub mains around the building.

I took a Ze reading and got 35.0 ohms so was convinced it was a TT system, however upon inspection I can see that there is a large earth wire coming from behind the transformer outside, clipped directly to the wooden pole and then goes into the ground. When I looked behind the green tar filled box inside, I see that there is a clamp with the main earth (25mm) wire approx 5inch long going to to the MET.

Has anyone come accross this before, I thought if the electricity board supplied the earth it should be under 1 ohm no the 35.0 ohms I am getting. I even did a few Zs readings at internal sockets which ranged from 35.2 to 35.7 the further away I got.

All bonding appears up to date with services been earthed directly into the MET.

Heeeelpp (please)

 

[Deleted member 9648]

Are all circuits RCD'd includindg distribution circuits ? If so could be a TT but it sounds like an unusual TNCS arrangement with a dodgy connection which needs very urgent attention.Without RCD's a L-E fault is going to sit there all day till someone gets a handful or the place goes up in smoke.

 

[eskimo39]

No RCD's on distribution circuits or sub mains. I did find 2 sockets with in built RCD's but these tripped as soon as I tried any Ze or Zs test on any part of the installation.

From the MET there is a feed going outside to an earth rod but this was not installed by supplier. However this does not seem to be doing much good either.

You mention a dogy connection but I am unsure where this could be, the earth wire I can see from the transformer appears to go straight underground, I assume this somehow is connected to the armour of the incomming phase lines as the clamp indoors is at the top of the armouring.

I cannot find any descriptions of this anywhere on the web.

Also if it is TNCS what Ze reading should be found and who's responsibility is it if it is not up to scratch?

Thanks wirepuller. Anyone else come across this before?

 

[Deleted member 9648]

If it was a TT you would expect something like a 300ma RCD protecting the entire installation and 30ma rcd's protecting sockets etc...if not present a live -earth fault is not going to clear with a Zs of 35ohms...You could try phoning the supplier as they should have information on whether they have supplied a PME connection...or whether one is available. Have to hand the full installation address including postcode,and the meter serial no.They will also want to know what you've had for breakfast for the last ten years,and be prepared for lots of being told ' i'ts not my department...ring this number' .... hanging on listening to greensleeves for half an hour and then being cut off. Good luck.

edit....cant remember off the top of my head the max ze for tncs and tns but i'ts less than 1ohm

 

[rocker]

TN-S is 0.8 ohms. TN-C-S (pme) is 0.35. That's for 1 phase domestic I don't know if it's different for 3 phase but I can't see why it would be.

Is the earth clamp loose or has the conductor underneath become dirty? You say the earth cable goes underground - any chance the groundsman's spade has gone through it?

 

[eskimo39]

Cheers for the help guys, I thought about damage to the incomming earth but have no real way of testing it as it is connected at the transformer and then straight into the ground. I am unsure whether I could strip a little of the pvc off and test between there and where it is clamped indoors.

I checked the connections indoors and it was fine. Consequently today I found a circuit which had a direct live-earth fault and nothing tripped.

Will try call the suppliers and see what help they can give.



UPDATE

Thanks again guys

Called YEDL and they arrived promptly. The guy did a basic test and got over 1 ohm. Funnily their meters don't have the facility to give an exact reading over 1 ohm.

We then shut the power off to the whole golf club which went down well. Disconnected all earths from MET and only tested the incomming earth. Again over 1 ohm so I asked if I could test with my meter. Got a reading of 40.7 ohms so YEDL refused to power back on and instead we are now waiting for their response team to arrive around 6pm tonight to investigate further.

Great